At present, LCD displays are often equipped with means for adjusting the contrast, whereby such adjustment is carried out at the beginning of a phase of use of the display. The contrast is not adjusted anymore during the use of the display.
JP-8292129 describes a separate measurement device which is put on the display at the moments when measurements are carried out only. Such a sensor for measuring display light has any one of an optical system, an aperture and an optical fibre which are mounted on a display surface. Among all the light beams emitted from the display surface, only the light corresponding to the optimum observing angle is applied into a sensor element. The sensor described does not allow for compactness and integration. The height of the sensor is large, as, in order to apply only the light corresponding to the optimum observing angle to the sensor element, the sensor element has to be placed quite some distance away from the aperture, and thus from the screen. The sensor also covers a quite large area. The sensor described is not designed for real-time use in parallel with the application displayed on the screen. For instance it would be impractical for a mobile phone display.
For some applications it is preferred to have a measurement device which can stay on the display all the time, so as to be able to continuously measure the light output, and thus continuously correct the light output and/or the colour of a displayed image.
From EP-0313331 an apparatus for controlling the brightness and contrast of a liquid crystal display during its actual use is known. A predetermined light level is produced and transmitted through an LCD panel by a light emitting diode. At the other side of the panel, the light transmitted through the panel is sensed by a sensor, and correspondingly a signal representative thereof is produced. This signal is monitored in real time and is used for adjusting in real time the grey scale levels of the panel. The light emitting diode and the sensor are positioned on opposite surfaces of the LCD panel, under a light shielding material. This light shielding material shields the viewer from the light generated in that portion of the screen, but also shields the sensor from ambient light. A disadvantage of this solution is that it cannot be retrofitted on existing screens. Furthermore, such a covered test pixel, although not having problems with ambient light, masks too much of the LCD.
U.S. Pat. No. 5,162,785 and DE-4129846 also describe a device for optimising the contrast of an LCD. The LCD panel comprises a display area and a separate test zone. A sensor observes the test zone. Both the sensor and the test zone are protected from ambient light by an optical mask.
U.S. Pat. No. 5,490,005 describes a light sensor placed on an own light source (LED) of a display device. A first disadvantage is that this cannot be implemented on commercially available LCD panels. A further disadvantage is that the feedback loop does not incorporate the backlight that is used for the active image content, and therefore this structure is not very precise: it does not measure the overall result of all image defining elements (such as the LCD itself, the backlight, filters, temperature etc.).